upload your picture to any sharing website, like imageshack.com or photobucket.com or anything similar.
copy the direct link to your image after upload,
paste it here in between this:
if your image address is
http://www.photobucket.com/someimage.jpg
it will look like this:
(just remove the spaces at the beginning and end of the image link)
press preview before you submit to make sure it works.